Knitted, braided, non-woven and woven textile materials, whether of natural of of synthetic fibrous material are usually porous. Over the past few years it has been found that a number of textiles of synthetic fibers are compatible with the human body, so that such textiles either as flat fabrics or in tubular form are suitable for use as patches or as replacements for portions of arteries or veins. Vascular prostheses constitute a particularly important use for such materials in that they can be used for repair and replacement of vessels in both humans and animals. Among the most important of such applications, are those in which the structure must be bifurcated.
For incorporation of such material into the human body or into an animal, the material structure must have certain physical characteristics as well as chemical characteristics. The chemical characteristics, of course, are those which influence rejection by the individual, whether human or animal, into which the structure is placed. Teflon has frequently been used. However, polyesters, and in particular the condensate of terephthalic acid with polyethylene glycol, have been found to be most satisfactory so far as chemical characteristics are concerned.
With respect to physical characteristics, the fabric must have a porosity great enough so that there can be ingrowth of fibrous tissue, a process which is termed fibrosis, so that the fabric can be completely covered with fibrous tissue characteristic of the healing process. In addition, the porosity must not be so great that excessive blood transfer or hemorrhaging therethrough will result in a hematoma or or collection or pooling of blood adjacent to the vessel.
As a step in preparing the fabric for implantation preclotting of blood on the fabric prior to implantation is generally carried out. Finally, tests have shown that the thickness of the fabric should be about 0.1 mm to 1.0 mm, approximately the thickness of that portion of the circulatory system to be repaired or replaced. Further characteristics which must be considered are flexibility, extensibility, strength and resilience sufficient to withstand blood pressure. Also, the construction of the fabric is preferably such that ravelling does not occur. This is most important when the graft is implanted into a vein or artery. Also, it is necessary that the fabric be capable of withstanding the conditions involved in sterilizing same.
As aforenoted, the porosity of the fabric is critical due to the fact that the body heals by fibrosis. Moreover, in considering the porosity of a fabric it is necessary that the uniformity of the porosity over the complete area of the structure be taken into account. As is evident, where the porosity varies very widely, then hemorrhaging could take place in one section of the structure although the average porosity of same would appear to be appropriate for implantation. So far as the usual woven or knitted fabric is concerned, where multi-filament thread is used, the flow through the fabric is a function of the spacing of the filaments which may or may not be crimped. As is evident, the degree of control achievable in the spacing between the filaments comprising a single strand of yarn is not great. Consequently, it is difficult to control the uniformity of porosity of a fabric which is woven or which is knitted by the usual techniques. In contrast, fabrics having a warp-knitted, lock-stitched or tricot-stitched construction can be made so that they present interstices which are polygons, usually three-sided, of relatively uniform size. Under such circumstances, the porosity of a fabric can be well controlled. However, even when knitted on Raschel machines using the finest needles and filament threads, i.e., yarns, the porosity of the resultant fabric prior to compaction is excessive so far as implantation is concerned.
The porosity of knitted material conventionally is presented as a measurement of the Wesolowski scale. To carry out the Wesolowski test, a piece of a test fabric is clamped against a flat surface having an orifice therein. Water is forced against the opposite side of the fabric at a constant pressure of 120 mm of mercury. The rate at which water passes through the fabric is expressed in ml/minute/cm.sup.2 of fabric. The value of 24,000 on the Wesolowski scale corresponds to free flow. For a vascular graft, the porosity of the fabric should be from about 30 to about 5,500, and preferably should lie between 1,200 and 4,000. Knitted fabric, whether of tubing or flat stock, even when knitted on the finest gauge double-needle bar Raschel machine available and with filament yarns, usually gives a reading in excess of 7,500 and may exceed 8,500 on the Wesolowski scale. Consequently, to reduce the porosity of the material, the fabric must be uniformly shrunk. The shrinkage in the wale direction of the fabric should be at least 30% and preferably should be at least about 40%.
In view of the increasing use of implanted fabric, particularly for vascular grafts, there is need for a process by which such compaction can be reliably carried out at relatively low cost. Furthermore, there is need for fabric having the properties described above.